CLI: Improvements to lxc config and lxc profile shell completions

[kadinsayani]

This PR includes updates to lxc/completion.go, improving the shell completion functionality for instance configuration keys.

Summary of changes:

• Refactorings that improve performance and correctness;
• Updates cmpInstanceKeys and cmpInstanceAllKeys functions to use the metadata API (resolves Missing instance config suggestions from tab completion #14537);
• Adds cmpInstanceSetKeys function for improved lxc config unset completions.

[simondeziel]

Thanks!

[tomponline]

@kadinsayani @simondeziel has any consideration been thought about wrt to caching the metdata api response for a particular remote so it doesnt have to keep re-requesting it (perhaps tied to the version number returned by the remote's /1.0 api)?

[kadinsayani]

@kadinsayani @simondeziel has any consideration been thought about wrt to caching the metdata api response for a particular remote so it doesnt have to keep re-requesting it (perhaps tied to the version number returned by the remote's /1.0 api)?

I'll include this with #14543 :)

[simondeziel]

@kadinsayani @simondeziel has any consideration been thought about wrt to caching the metdata api response for a particular remote so it doesnt have to keep re-requesting it (perhaps tied to the version number returned by the remote's /1.0 api)?

That's an interesting idea, thanks for suggesting! I'd probably tie it to the server_pid so that even minor snap updates also invalidate the cache.